Method of cleaning metal strip continuously



March 6, 1962 R. H. SHOEMAKER ETAL METHOD OF CLEANING METAL STRIP CONTINUOUSLY Filed Feb. 19, 1960 IN V EN TORS ROBERT H. SHOEMAKER y JOHN A. FALER ATTORNEYS Robert H. Shoemaker and John A. Faler, Detroit, Mich, assignors to Kolene Corporation, Detroit, Mich. Filed Feb. 19, 1960, Ser. No. 9,775 2 Claims. (Cl. 13415) This application relates to method of cleaning metal strip continuously and generally relates to apparatus of the character disclosed in the Dunlevy Patent No. 2,635,- 062 of April 1-4, 1953. That patent shows a metal strip passing through a molten salt bath and emerging between two guide and wiping exit rollers which direct the exiting strip and also function for squeezing off excess molten salt from the emerging strip to maintain salt dragout losses at a minimum.

The salt is of the formula of another Patent No. 2,458,- 661, namely one part by weight of alkali metal nitrate, 1.5-3.0 parts by weight of alkali metal hydroxide, and 0.1 part by weight of alkali metal chloride maintained in molten form.

Despite the fact that these rolls are cast iron, and have the advantages set forth in the Dunlevy patent, nevertheless, and though the substitution of cast iron for steel for these exit directing and wiping rollers does and did function to reduce scratching to a considerable extent and yet maintain dragout losses to a satisfactory level, we have in our studies of the apparatus of the Dunlevy patent continuously sought ways to reduce exit and directing roll scratching to an even greater extent while preserving an acceptable minimum of dragout loss.

We have concluded from our studies that the greater factors causing scratching arose from the presence of the upper one of these guide rollers. We have concluded that the salt film on the upper side of the emerging strip cools somewhat more rapidly than the salt film on the lower side and consequently a greater quantity of cooled abrasive particles were formed on the upper side of the strip and consequently a greater proportion of scratching took place on the upper side of the strip. We found that the scratching on the lower side of the strip was considerab-ly less and We have concluded that this was due to the fact that the salt film on the lower side of the strip not only quickly reduced itself in quantity by dropping ofl the strip as the strip moved out of the bath, due to the influence of gravity and movement of the strip, but also the salt film on the under side of the strip remained hot and molten to a greater extent than the film on the upper side of the strip, possibly due to a greater heating effect on the underside of the strip due to the proximity of the molten salt bath immediately below the strip. Hence, we concluded that it was the presence of the upper roll as contrasted from the lower roll that proved to be the source of undesirable scratching.

We have considered eliminating both rolls, but this was not feasible since the lower roll was needed for directing the emerging strip.

We have tried eliminating the upper one of the rolls and while this reduced scratching on the upper surface of the strip, it nevertheless permitted an excessive salt dragout and proved to be not an economical method of operation.

We have also tried heating the rollers and also heating the strip at the rollers by heating means above the strip, but for many reasons this expedient did not prove entirely satisfactory.

As a result of the continuation of our prolonged efforts to reduce scratching to a minimum while maintaining salt dragout losses at an acceptable level, we have developed and disclose here in combination a lower roll and a hot gas 3,fi24,139 Patented Mar. 6, 1%62 blast means, both provided at the exit end of the bath. We have discovered that an acceptable gas blast could be in the form of hot air or steam and we have discovered that such gas blasting in this combination functions to eliminate the nip and to reduce scratching and yet without resulting in excessive salt dragout, while preserving the means to direct the strip out of the bath.

Our problem was analyzed by us to be the discovery of some arrangement for preventing excessive dragout loss and yet avoid excessive scratching particularly of the upper surface of the strip, while preserving the directing of the strip; or setting forth this in another way, our problem was to prevent scratching of the upper surface of the strip and yet avoid excessive dragout loss, while directing the exiting strip.

The solution to our problem at present seems to be the substitution of a lower roll plus a hot gas blast at the location of and in substitution for the two exit end nut guide rolls of the apparatus shown in the Dunlevy patent.

A more specific description of our invention will be set forth in the appended specification which relates to the annexed drawing.

In this drawing:

FIG. 1 is a diagrammatic side view of a part of a continuous strip treating apparatus including a molten salt bath.

FIG. 2 is a diagrammatic top plan View of the apparatus of FIG. 1.

Referring now to the drawing, it will be observed that the drawing shows at 10 a continuously moving strip being treated continuously by the process of the aforesaid patent. The strip 10 is first heated in a furnace and then enters the molten salt bath 11 in tub 12, the bath being of the molten alkali salt type as set forth in said patents with the salt being maintained molten by suitable heating means. As strip 10 goes through the bath, it passes over an entrance guide roll 14, and under immersed rolls l5.

Disposed on a horizontal axis at the point where the strip emerges from the bath is an immersed roller 20 which directs the steel strip out of the. bath and towards other rollers not shown which direct the strip under a water spray means and through a water rinse bath and an acid rinse bath all as set forth in said patent.

As set forth in Patent No. 2,635,062, roll 20 is of cast iron. The action of the molten salt carried by the emerging strip 10 to roller 20 removes graphitic carbon from the cast iron roller 20 and leaves minute fissures on roll 29 which provide reservoirs or pockets for the fluid or molten salt on the roll 20 which functions as a lubricant on the roll and to a considerable extent prevents the roll from scratching the polished stainless steel strip passing by and engaging it. Roll 20 also functions for squeezing off excess molten salt from the under surface of strip 10. Roll 20 also directs the emerging strip 10.

To maintain roll 26 and the emerging strip 10 and the salt film on both surfaces of the strip in as nearly complete a molten and fluid condition as possible suitable heating means not shown may be used to heat roll 20 and strip 10 at such roll. The close proximity of the emerging strip 10 and roll 20 to the hot salt bath 11 also aids in maintaining the salt film. on the emerging strip in molten condition.

What we have described heretofore however is simply the apparatus of the Dunlevy patent. We now turn to the specific improvement of this application.

Whereas the Dunlevy patent shows two rolls at the point where the strip emerges from the bath, in this application we show but one roll 20, a lower one over which the strip passes and we omit the upper one of the rolls.

Thus, it will be observed that nothing exists to cause scratching of the upper surface of the strip as it leaves the bath with a molten salt film on it and particularly on its upper surface. To remove this molten upper surface film and return it to the bath, yet without causing scratching of the upper surface, we provide a gas blast means in the form of gas blasting nozzles 100 disposed and located as shown and functioning to blast a stream of heated gas in the direction shown to impinge upon the molten salt film on the upper surface of the strip and direct it back into the bath where it can be brought again to the operating bath temperature and used as part of the cleaning bath.

The characteristics of the gas blast at 100 are as follows:

(1) In one example, it is a blast of hot air at a high temperature of 900 F. or more.

(2) In another example, it is a blast of high temperature steam, at a temperature of approximately 900 F. or even higher.

(3) Other hot inert gases may be considered.

A requirement is that the gas blast at 100 be of a sufficiently high temperature that it will not cause solidifcation of the molten salt film or strip 10 nor unduly increase its viscosity.

Another requirement is that the gas be inert to the salt and the strip. Ordinary air or steam at high temperature have been found satisfactory.

The blasting means may be made of separate nozzles or it may be made as shown of a single duct 101 having a continuous nozzle slot 100 extending laterally across the strip.

We have discovered through considerable experimentation that our substitution of a gas blast means for the top wiper roll of the Dunlevy patent has resulted in considerable reduction of scratching and in fact almost complete elimination of scratching from the upper surface of the stainless steel strip emerging from the bath and yet has preserved salt dragout losses to an acceptable minimum.

Now having set forth the apparatus herein disclosed, we refer to the claims which follow.

We claim:

1. The method of handling metal strip without scratching or marring the surface thereof in continuous passage through a molten salt bath comprising alkali metal salts which tend to solidify into small abrasive particles when subjected to temperatures below their melting point, comprising passing the strip through a liquid bath comprising said molten alkali metal salts and removing the strip supported and guided over an end of the bath by a single cast iron roller wet with molten salts of said bath, and continuously removing excess molten salt from the top surface of said strip by a hot gas blast directed thereon.

2. The method as defined in claim 1 wherein the gas is selected from the group consisting of hot air and steam.

References Cited in the file of this patent UNITED STATES PATENTS 2,195,565 Moss Mar. 26, 1940 2,288,980 Turin July 7, 1942 2,314,369 Reed Mar. 23, 1943 2,529,762 Brisse Nov. 14, 1950 2,635,062 Dunlevy Apr. 14, 1953 2,818,075 Dunlevy Dec. 31, 1957 2,967,119 Gutterman Ian. 31, 1961 

1. THE METHOD OF HANDLING METAL STRIP WITHOUT SCRATCHING OR MARRING THE SURFACE THEREOF IN CONTINUOUS PASSAGE THROUGH A MOLTEN SALT BATH COMPRISING ALKALI METAL SALTS WHICH TEND TO SOLIDIFY INTO SMALL ABRASIVE PARTICLES WHEN SUBJECTED TO TEMPERATURES BELOW THEIE MELTING POINT, COMPRISING PASSING THE STRIP THROUGH A LIQUID BATH COMPRISING SAID MOLTEN ALKALI METAL SALTS AND REMOVING THE STRIP SUPPORTED AND GUIDED OVER AN END OF THE BATH BY A SINGLE CAST IRON ROLLER WET MOLTEN SALTS OF SAID BATH, AND CONTINUOUSLY REMOVING EXCESS MOLTEN SALT FROM THE TOP SURFACE OF SAID STRIP BY A HOT GAS BLAST DIRECTED THEREON. 